Graphene-based bolometer

ABSTRACT

A bolometer. In one embodiment a graphene sheet is configured to absorb electromagnetic waves. The graphene sheet has two contacts connected to an amplifier, and a power detector connected to the amplifier. Electromagnetic power in the evanescent electromagnetic waves is absorbed in the graphene sheet, heating the graphene sheet. The power of Johnson noise generated at the contacts is proportional to the temperature of the graphene sheet. The Johnson noise is amplified and the power in the Johnson noise is used as a measure of the temperature of the graphene sheet, and of the amount of electromagnetic wave power absorbed by the graphene sheet.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a continuation in part of U.S. applicationSer. No. 14/794,591, filed Jul. 8, 2015, entitled “GRAPHENE-BASEDINFRARED BOLOMETER”, the entire content of which is incorporated hereinby reference, which claims priority to and the benefit of U.S.Provisional Application No. 62/181,177, filed Jun. 17, 2015, entitled“GRAPHENE-BASED INFRARED BOLOMETER”, the entire content of which isincorporated herein by reference.

BACKGROUND 1. Field

One or more aspects of embodiments according to the present inventionrelate to detection of infrared light or microwaves, and moreparticularly to a high-sensitivity, high-bandwidth bolometer fordetecting infrared light or microwaves.

2. Description of Related Art

Bolometers have multiple applications, including applications insensitive imaging systems and in communications systems. Constructing abolometer with good sensitivity for wavelengths in the range spanningfrom 10 microns or more to 1 micron may be challenging, in part becausethe thermal inertia of sensing elements of related art bolometers may besufficient to impair their bandwidth and to reduce their sensitivity.Both bandwidth and sensitivity may be important for communicationsapplications; such applications may use a wavelength of 1550 nm.Sensitivity may also be important for imaging applications, andbandwidth may be important for high-speed imaging. Thus, there is a needfor a high-sensitivity, high-bandwidth bolometer.

SUMMARY

Aspects of embodiments of the present disclosure are directed toward aninfrared or microwave bolometer. In one embodiment a graphene sheet isconfigured to absorb electromagnetic waves. The graphene sheet has twocontacts connected to an amplifier, and a power detector connected tothe amplifier. Electromagnetic power in the evanescent electromagneticwaves is absorbed in the graphene sheet, heating the graphene sheet. Thepower of Johnson noise generated at the contacts is proportional to thetemperature of the graphene sheet. The Johnson noise is amplified andthe power in the Johnson noise is used as a measure of the temperatureof the graphene sheet, and of the amount of electromagnetic wave powerabsorbed by the graphene sheet.

According to an embodiment of the present invention there is provided abolometer including: a graphene sheet having a first pair of contactsand configured: to be coupled to received electromagnetic waves; to havea temperature, when electromagnetic power in the receivedelectromagnetic waves is absorbed by the graphene sheet, correspondingto the amount of electromagnetic power absorbed by the graphene sheet;and to generate thermal noise at the first pair of contacts at a levelcorresponding to the temperature a Fabry-Perot resonator including twomirrors, the graphene sheet being between the two mirrors; and a circuitconnected to the first pair of contacts, the circuit configured tomeasure the thermal noise level.

In one embodiment, the bolometer includes a refrigerator configured tocool the graphene sheet to a temperature below 4 K.

In one embodiment, the refrigerator is a pulse tube refrigerator.

In one embodiment, the refrigerator is a Gifford-McMahon cooler.

In one embodiment, the graphene sheet substantially has the shape of arectangle, the rectangle having a length and a width, the length beinggreater than or equal to the width.

In one embodiment, the length of the rectangle is less than 20 microns.

In one embodiment, the product of the length of the rectangle and thewidth of the rectangle is less than 1000 square microns.

In one embodiment, the graphene sheet has an electron mobility of morethan 100,000 cm2/V/s.

In one embodiment, the bolometer includes a first layer of hexagonalboron nitride immediately adjacent to a first surface of the graphenesheet, and a second layer of hexagonal boron nitride immediatelyadjacent to a second surface of the graphene sheet.

In one embodiment, each of the first layer of hexagonal boron nitrideand the second layer of hexagonal boron nitride has a thickness greaterthan 4 nm and less than 40 nm.

In one embodiment, the circuit includes an amplifier connected to thefirst pair of contacts.

In one embodiment, the bolometer includes a matching circuit connectedbetween the first pair of contacts and the amplifier.

In one embodiment, the bolometer includes a power detector connected tothe amplifier.

In one embodiment, the graphene sheet consists of a single atomic layerof graphene.

In one embodiment, the graphene sheet includes two atomic layers ofgraphene.

According to an embodiment of the present invention there is provided animaging system including an array of bolometers, each of the bolometersof the array having an electromagnetic wave input, the electromagneticwave inputs forming an array of electromagnetic wave inputs; and animaging system configured to project an image onto the array ofelectromagnetic wave inputs.

According to an embodiment of the present invention there is provided abolometer including: a graphene sheet having a first pair of contactsand a second pair of contacts and being configured: to be coupled toreceived electromagnetic waves; to have a temperature, whenelectromagnetic power in the received electromagnetic waves is absorbedby the graphene sheet, corresponding to the amount of electromagneticpower absorbed by the graphene sheet; and to generate thermal noise atthe first pair of contacts at a level corresponding to the temperature;and a circuit connected to the first pair of contacts, the circuitconfigured to measure the thermal noise level, wherein a first contactof the second pair of contacts, the graphene sheet, and a second contactof the second pair of contacts together form a part of a microstriptransmission line.

In one embodiment, the bolometer includes a second end of the microstriptransmission line, forms a quarter-wave open stub connected to thegraphene sheet.

According to an embodiment of the present invention there is provided abolometer including: a graphene sheet having a first pair of contactsand a second pair of contacts and being configured: to be coupled toreceived electromagnetic waves; to have a temperature, whenelectromagnetic power in the received electromagnetic waves is absorbedby the graphene sheet, corresponding to the amount of electromagneticpower absorbed by the graphene sheet; and to generate thermal noise atthe first pair of contacts at a level corresponding to the temperature;and a circuit connected to the first pair of contacts, the circuitconfigured to measure the thermal noise level, wherein each contact ofthe second pair of contacts is connected to a respective conductor of apair of conductors of a log periodic antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects, and embodiments are described in conjunction with theattached drawings, in which:

FIG. 1 is conceptual diagram illustrating the operation of an infraredbolometer, according to an embodiment of the present invention;

FIG. 2 is a block diagram of an infrared bolometer according to anembodiment of the present invention;

FIG. 3A is a perspective view of a bolometer sensor assembly accordingto an embodiment of the present invention;

FIG. 3B is a schematic plan view of a bolometer sensor assemblyaccording to an embodiment of the present invention;

FIG. 3C is a perspective view of a communications system includingmobile transceivers according to an embodiment of the present invention;

FIG. 3D is a block diagram of an imaging system according to anembodiment of the present invention;

FIG. 3E is a schematic side view of a graphene sheet sandwiched betweentwo layers of hexagonal boron nitride, according to an embodiment of thepresent invention;

FIG. 4 is a block diagram of a Johnson noise measuring circuit accordingto an embodiment of the present invention;

FIG. 5 is a schematic diagram of a front end circuit according to anembodiment of the present invention;

FIG. 6A is a block diagram of a thermal model of a graphene sheetaccording to an embodiment of the present invention; and

FIG. 6B is a graph of the thermal conductance of a graphene sheet as afunction of temperature according to an embodiment of the presentinvention.

FIG. 7A is a hybrid schematic diagram and plan view of a portion of abolometer, according to an embodiment of the present invention;

FIG. 7B is a hybrid schematic diagram and plan view of a portion of abolometer, according to an embodiment of the present invention;

FIG. 8A is a plan view of a portion of a bolometer, according to anembodiment of the present invention;

FIG. 8B is a plan view of a portion of a bolometer, according to anembodiment of the present invention; and

FIG. 8C is a plan view of a portion of a bolometer, according to anembodiment of the present invention.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments of agraphene-based infrared bolometer provided in accordance with thepresent invention and is not intended to represent the only forms inwhich the present invention may be constructed or utilized. Thedescription sets forth the features of the present invention inconnection with the illustrated embodiments. It is to be understood,however, that the same or equivalent functions and structures may beaccomplished by different embodiments that are also intended to beencompassed within the spirit and scope of the invention. As denotedelsewhere herein, like element numbers are intended to indicate likeelements or features.

Referring to FIG. 1, in one embodiment, an infrared bolometer includes agraphene sheet 120, configured to absorb infrared electromagnetic waves105. Electrons in the graphene sheet are weakly coupled to phonons inthe graphene sheet, and the mechanisms by which heat escapes theelectrons of the graphene sheet may provide heat paths with a relativelylow total thermal conductivity. As a result, even for relatively littleabsorbed infrared electromagnetic power, the temperature of theelectrons in the graphene sheet may increase significantly, as shownconceptually by the illustration of thermometer 122.

Referring to FIG. 2, in one embodiment, the infrared bolometer includesan infrared waveguide 110 and a graphene sheet 120 forming a bolometersensor assembly 124, and a thermal noise or “Johnson noise” measuringcircuit 130 for monitoring the Johnson noise generated by the graphenesheet 120. When infrared electromagnetic waves propagate in thewaveguide 110, evanescent waves outside the waveguide 110 overlap with,and couple to, the graphene sheet 120. This coupling allows power to beabsorbed from the electromagnetic waves by the graphene sheet 120,raising the temperature of the graphene sheet 120 to a temperaturecorresponding to the amount of electromagnetic power absorbed by thegraphene sheet 120. The temperature may be measured by measuring theJohnson noise (i.e., the thermal noise) produced by the graphene sheet120.

When power is absorbed from infrared electromagnetic waves by thegraphene sheet 120, the absorption occurs primarily through interactionof the electromagnetic waves with the electronic degrees of freedom ofthe graphene sheet 120, because the interactions between theelectromagnetic waves and the nuclei of the graphene sheet 120 aresignificantly weaker than the interactions between the electromagneticwaves and the electrons of the graphene sheet 120. Electrons in thegraphene sheet 120 are weakly coupled to phonons in the graphene sheet120, and, in part because of this, the total thermal conductivitycorresponding to mechanisms by which the electrons may lose heat (e.g.,coupling through the contacts, coupling to the lattice, and coupling tothe electromagnetic environment) is relatively small. As a result, theabsorption of power from the electromagnetic waves results in arelatively high electron temperature, which in turn results in arelatively high Johnson noise level (i.e., a relatively high thermalnoise level) in the potential between any pair of spatially separatedpoints on the graphene sheet 120. The Johnson noise may be measured attwo contacts 330, disposed, for example, at two ends of a rectangulargraphene sheet 120 (FIG. 3A).

In some embodiments, the infrared waveguide 110 may be absent, andinfrared radiation (e.g., infrared electromagnetic waves propagating infree space) may instead illuminate the graphene sheet 120 directly, orthe graphene sheet 120 may be placed between parallel mirrors forming aFabry-Perot resonator, and received infrared radiation (e.g., infraredelectromagnetic waves propagating in free space) may be resonantlyamplified, resulting in higher infrared irradiance at the graphene sheetthan in the received infrared radiation. In other embodiments,illustrated for example in FIGS. 7A-8C and described in further detailbelow, a bolometer may detect electromagnetic waves having longerwavelengths than those of infrared radiation, e.g., microwaves. As usedherein, the term “microwave” includes the portion of the electromagneticspectrum suitable for receiving with an antenna and guiding withconductors, and includes frequencies in the range from 1 MHz to 1 THz.Photons in such microwave electromagnetic waves may be absorbed by thegraphene sheet 120, producing a signal in a manner similar to thatdescribed above for infrared radiation.

Referring to FIG. 3A, the waveguide 110 and the graphene sheet 120 ofthe bolometer sensor assembly 124 may be fabricated as an integratedcomponent on a substrate 310. The waveguide 110 may be fabricated by anyof various methods known in the art for fabricating a waveguide 110 onthe surface of a substrate 310. In one embodiment, a layer of silicondioxide is formed on a silicon substrate 310, and patterned usingphotolithography to form the waveguide 110 as a raised structure. Alayer 320 of silicon nitride may then be formed over the waveguide 110and the surrounding area, so that the waveguide 110 core has siliconnitride on both sides of it and above it. This structure may then bepolished, so that the upper surface of the structure is flat and smooth.In other embodiments, the waveguide 110 may be formed by depositing alayer of silicon dioxide on a silicon substrate 310, depositing a layerof silicon on the silicon dioxide, and patterning the silicon layerusing photolithography to form the core of the waveguide 110 as a raisedsilicon structure. This structure may then be planarized, i.e., madeflat and smooth, by depositing an additional layer of silicon dioxideand polishing it down to the top surface of the raised siliconstructure. In other embodiments the waveguide 110 may be composed ofanother first material, surrounded by one or more other materials havinga lower index of refraction than the first material. The resultingwaveguide structure may have a thickness of between 50 nm and 2000 nm;in one embodiment it has a thickness of between 100 nm and 1000 nm. Thetransverse dimensions of the waveguide structure may be somewhat smalleror considerably smaller than the wavelength of the infrared light to bedetected. The waveguide 110 may be single-mode or multi-mode whenguiding light at the wavelength of the infrared light to be detected.The substrate may be substantially flat, e.g., within 1 micron of beingflat over the area including the waveguide, and the waveguide may beformed, e.g., using one of the processes described above, in a layerhaving a thickness greater than 50 nanometers and less than 2 microns.The front end of the waveguide 110 may extend to the edge of thesubstrate 310 as shown, and off-chip coupling optics 340 may be used tolaunch infrared light into the waveguide 110. In other embodimentsportions of the coupling optics may be fabricated on the substrate 310.In some embodiments the waveguide 110 is omitted and infrared lightpropagating in free space illuminates the graphene sheet 120 directly.

The waveguide 110 may be straight, and, to increase the amplitude of theevanescent waves overlapping the graphene sheet 120, it may be part ofan optical resonator, constructed, for example, by forming a reflector(e.g., a Bragg reflector) at each end of a section of the waveguide 110.Bragg reflectors may be formed by creating periodic defects in oradjacent to the waveguide 110, e.g., by forming holes in or adjacent thewaveguide structure with a focused ion beam. The reflector at the frontend of the waveguide 110 (i.e., the end first encountered by thearriving infrared light) may be partially reflective, to allow theinfrared light to enter the resonator, and the reflector at the otherend (the “back” end) of the waveguide 110 may be highly reflective, toavoid allowing light to escape from the back end of the waveguide 110.In some embodiments only one reflector is used, at the back end of thewaveguide 110.

In other embodiments the waveguide 110 may not be straight, but may haveone or more curves, increasing the length of the section of waveguide110 that is adjacent to the graphene sheet 120, and from whichevanescent waves may interact with the graphene sheet 120. A curvedsection in the waveguide may have a radius of curvature less than thelength of the graphene sheet, and in the curved section the direction ofthe waveguide may change by 45 degrees or more. The increased length ofthe section of waveguide 110 adjacent to the graphene sheet 120 mayincrease the fraction of the electromagnetic energy launched into thewaveguide 110 that is absorbed by the graphene sheet 120. The waveguide110 may have a double spiral shape as illustrated in FIG. 3A. In otherembodiments the waveguide 110 may have the shape of a single spiral,with the back end of the waveguide in the center of the spiral. The backend of the waveguide 110 may be at an edge of the substrate 310 asillustrated in FIG. 3A, or it may be elsewhere on the substrate 310,e.g., near the middle in the case of a waveguide 110 in the shape of asingle spiral.

Referring to FIG. 3B, in one embodiment the waveguide 110 may have ameandering shape, covering a region that roughly corresponds to theextent of the graphene sheet 120 as illustrated.

In yet other embodiments, the waveguide 110 may have one or more curvesand also form part of a resonator, to further increase the fraction ofthe electromagnetic energy launched into the waveguide 110 that isabsorbed by the graphene sheet 120.

Infrared light may be launched into the waveguide 110 by any of severalsystems known to those of skill in the art. For example, referring toFIG. 3C, transceivers using coupling to free-space propagating waves maybe used for mobile communications e.g., between ships 380 and a tower385. In such an application, the infrared light may be coupled into thewaveguide 110 using one or more suitable powered optics, such as lensesor curved mirrors. In other communications applications, if the infraredlight to be detected is initially propagating in an optical fiber, itmay be launched into the waveguide 110 of the bolometer using anysuitable one of a variety of fiber-to-chip couplers known to those ofskill in the art.

Referring to FIG. 3D, in one embodiment an infrared imaging system mayemploy an array of bolometer sensor assemblies 124 constructed accordingto an embodiment of the present invention. Each of the bolometer sensorassemblies 124 has an optical input (e.g., the input of the waveguide110, or the input of coupling optics 340, or the input of a fibercoupled to the waveguide 110). The optical inputs form an array that isilluminated through imaging optics that project an image onto the arrayof optical inputs. The array of bolometer sensor assemblies 124 is readout by a set of readout electronics, which may for example be an arrayof Johnson noise measuring circuits 130. In one embodiment the imagingoptics include an array of coupling lenses to couple light into thewaveguides 110 of the array of bolometer sensor assemblies 124, or tocouple light into a corresponding array of fibers, each of which iscoupled to a respective bolometer sensor assembly 124 in the array ofbolometer sensor assemblies. In other embodiments an analogous imagingsystem (e.g., a microwave reflector) may be used to form microwaveimages on a bolometer array.

The graphene sheet 120 may be a single-layer sheet, i.e., it may be oneatomic layer thick, or it may be a multi-layer graphene sheet 120,having, e.g., 2, 3, 4, or more layers. Referring to FIG. 3E, in oneembodiment, the graphene sheet 120 is encapsulated in hexagonal boronnitride (hBN). As is known to those of skill in the art, a sandwich isformed, with the graphene sheet 120 sandwiched between two layers 290 ofhexagonal boron nitride. Each layer 290 of hexagonal boron nitride maybe between 4 nm and 40 nm thick; these layers 290 of hexagonal boronnitride may keep the surface of the graphene sheet 120 clean, i.e., theymay prevent surface contamination from compromising the properties ofthe graphene sheet 120. The sandwich, composed of the two outer layers290 of hexagonal boron nitride encapsulating the graphene sheet 120, maythen be disposed on the portion of the substrate 310 that includes thewaveguide 110.

Each hexagonal boron nitride layer 290 may be a single crystal, with anatomically flat surface facing the graphene sheet 120. Each hexagonalboron nitride layer 290 may be annealed, e.g., at 250° C. for 10-15minutes, before the sandwich is assembled. The sandwich may be formed byfirst bringing a first layer 290 of hexagonal boron nitride into contactwith the graphene sheet 120, resulting in adhesion of the graphene sheet120 to the hexagonal boron nitride by van der Waals forces, and thenbringing the graphene sheet 120, on the first layer 290 of hexagonalboron nitride, into contact with the second layer 290 of hexagonal boronnitride, resulting in adhesion, again by van der Waals forces, at theinterface between the graphene sheet 120 and the second layer 290 ofhexagonal boron nitride. The edges of the sandwich may then be etched,e.g. using plasma etching, so that the edges of the two layers 290 ofhexagonal boron nitride and the edges of the graphene sheet 120 in thestructure remaining after the etch process coincide (i.e., are aligned).

The graphene sheet 120 may be rectangular as illustrated in FIG. 3A,with a length and a width, the length being greater than or equal to thewidth. The total area of the graphene sheet 120 may be less than 1000square microns. In one embodiment the graphene sheet 120 is about 10microns by 10 microns. In one embodiment the graphene sheet 120 has alength in the range 1.0-100.0 microns and a width in the range 1.0-100.0microns. An electrical contact 330 may be provided at each of twoopposing sides or ends of the graphene sheet 120. In one embodiment,contact is made with the edge of the graphene sheet 120, using a layerof metal deposited onto the edge of the sandwich.

For good performance, the graphene sheet 120 may be made as small aspossible, kept as clean as possible, and operated at as low atemperature as possible. In one embodiment, the graphene sheet 120 iscooled to 4 K, using, for example, a pulse tube refrigerator or aGifford-McMahon (GM) cooler. In other embodiments direct cooling withliquid helium, or with liquid helium in a partial vacuum (e.g., using a1 K pot, to reach temperatures below 4 K) may be used to cool thegraphene sheet 120.

The Johnson noise power at the two contacts may be proportional to theelectron temperature of the graphene sheet 120. As used herein, thetemperature of the graphene sheet 120 refers to the temperature of theelectrons in the graphene sheet 120; when electromagnetic power in theevanescent waves of the waveguide 110 is absorbed by the graphene sheet120, the electron temperature may differ from the phonon temperature.

In one embodiment, referring to FIG. 4, a Johnson noise measuringcircuit 130 includes a front end circuit 410 for amplifying the Johnsonnoise and a power detector 420 that converts the noise signal to a powersignal or temperature signal having a value (e.g., a voltage)corresponding to (e.g., proportional to) the noise power. Referring toFIG. 5, the front end circuit 410 may include an amplifier 510 that mayinclude a quantum noise limited amplifier followed by a high electronmobility transistor (HEMT) amplifier. The front end circuit 410 may alsoinclude a matching network, e.g., an inductor-capacitor (LC) matchingnetwork, for transforming the impedance of the graphene sheet 120, whichmay be about 1,000 ohms, to the input impedance of the amplifier 510(which may be about 50 ohms). In one embodiment the amplifier 510 has abandwidth of about 80 MHz around a frequency of about 1.1 GHz, and thematching network is tuned for a frequency of 1.1 GHz. In someembodiments the quantum noise limited amplifier may be a radio frequencysuperconducting quantum interference device (RF SQUID) amplifier, or itmay be a travelling wave parametric amplifier, or a tuned systemparametric amplifier (TPA), or any other kind of amplifier with asuitable frequency response that is quantum noise limited or nearlyquantum noise limited. In some embodiments the amplifier 510 does notinclude quantum noise limited amplifier, and has a HEMT amplifier as thefirst stage instead.

The front end may also include components and connections that may beused for diagnostics, e.g., during manufacturing, operation, or service.A bias tee may be used, for example, to drive a low-frequency currentthrough the graphene sheet 120, modulating its temperature, and thepresence of a corresponding modulation at the output of the powerdetector may then be used to verify the functioning of the device. Thedifferential thermal conductance of the graphene sheet 120 may also bemeasured in this manner. A directional coupler may be used to supplymicrowave power to the graphene sheet 120, while monitoring the outputof the power detector; this microwave power is essentially entirelyabsorbed, and this technique may be used to measure the differentialthermal conductance as well. A circulator may be used at the input ofthe amplifier 510 to prevent reflections, backwards-propagatingamplifier noise, or signals travelling in reverse through the amplifier510, from heating the graphene sheet 120.

The power detector of FIG. 4 may be a circuit for producing an outputsignal proportional to the total power at its input. It may include, forexample, a Schottky diode biased (e.g., with a bias tee) so that, inaddition to the bias current, it conducts a low-frequency currentapproximately proportional to the square of the microwave signal appliedacross its terminals. This low-frequency current may then be measured(e.g., as a change in the bias current). In other embodiments the powerdetector may be constructed according to another of various powerdetector circuits known to those of skill in the art. The output of thepower detector need not be proportional to the input power, and may, forexample, be a nonlinear function of the input power. The output of theJohnson noise measuring circuit 130 may be the (e.g., analog) output ofthe power detector 420, or a sampling circuit (e.g., an analog todigital (A/D) converter) may be connected between the output of thepower detector 420 and the output of the Johnson noise measuring circuit130, so that the output of the Johnson noise measuring circuit 130 is adigital data stream. In one embodiment the sampling rate of such an A/Dconverter is about 50 MHz.

The sensitivity of the bolometer may depend on the size of the graphenesheet 120, its cleanliness, and its temperature. Referring to FIG. 6A, athermal model for the graphene sheet 120 in the bolometer sensorassembly 124 may include the thermal mass (i.e., the heat capacity) ofthe graphene sheet 120, the electromagnetic power 105 heating thegraphene sheet 120, and a thermal conductance 605 to a thermal reservoir610. The smaller the heat capacity, the greater the bandwidth of thebolometer sensor assembly 124, and the smaller the thermal conductance605, the higher the steady-state temperature for a given amount ofelectromagnetic power 105 heating the graphene sheet 120. Referring toFIG. 6B, in the clean graphene limit, the thermal conductance G_(ep)(between the electrons and the phonons of the graphene sheet 120) may begiven by

G_(ep)=4Σ_(c)AT³

where T is the temperature, A is the area of the graphene sheet 120, andΣ_(c) is the electron-phonon coupling constant in the clean graphenelimit. This coupling constant Σ_(c) is independent of the temperature Tand of the area A of the graphene sheet 120, but increases withincreasing impurity density level, i.e., with decreasing electronmobility. For disordered graphene, the thermal conductance G_(ep)(between the electrons and the phonons of the graphene sheet 120) may begiven by

G_(ep)=3Σ_(d)AT²

where Σ_(d) is the electron-phonon coupling constant for disorderedgraphene.

The graph of FIG. 6B shows the expected thermal conductance G_(ep) for agraphene sheet 120 with an area of 100 square microns (e.g., a 10 micronby 10 micron square graphene sheet 120). The sensitivity of a bolometerusing a graphene sheet with the characteristic of FIG. 6B may beestimated by multiplying the thermal conductance by the noise of theamplifier 510 (FIG. 5). For example, for amplifier noise of 1 mK/sqrt(Hz), and for G_(ep)=10 pW/K (e.g., from FIG. 6B, for a graphene sheet120 at 1 K), the expected sensitivity is 10 fW/sqrt (Hz).

In some embodiments, as mentioned above, a graphene sheet may be used todetect microwave electromagnetic radiation. Referring to FIG. 7A, in oneembodiment the electromagnetic radiation may be coupled (e.g., by asuitable receiving antenna, such as a horn antenna or a patch antenna)to an input transmission line 805, and the resulting guided waves maypropagate toward the graphene sheet 120 on the input transmission line805 (e.g., a microstrip transmission line), through an impedancematching network 810, and through a detector transmission line 815 (thatincludes the graphene sheet 120), to ground 820. The detectortransmission line 815 is illustrated, in FIG. 7A, in part schematically(as a line) and in part in a plan view, as a conductor havingsubstantially constant width. When the received electromagnetic wavesgenerate current in the graphene sheet 120, the electron temperature ofthe graphene sheet may increase, resulting in an increase in Johnsonnoise as described above. A Johnson noise measuring circuit 130 may beconnected to the two contacts 330 to detect such a temperature increase.Chokes 825 may be used to isolate the detector transmission line 815from the Johnson noise measuring circuit 130 over the range offrequencies of the electromagnetic radiation to be detected. Contactsbetween the detector transmission line 815 and the graphene sheet 120may be formed at two respective edges of the graphene sheet 120, andcontacts 330 through which the graphene sheet 120 is connected to theJohnson noise measuring circuit 130 may be at two other respective edgesof the graphene sheet 120, as illustrated in FIG. 7A.

Referring to FIG. 7B, in an embodiment similar to that of FIG. 7A, thereceived electromagnetic waves propagating on the input transmissionline 805 may be capacitively coupled to the detector transmission line815 by a coupling capacitor 830 (which may be a gap, as shown, in amicrostrip transmission line). A half-wave (or “lambda-over-two”)microwave resonator 835 with the graphene sheet 120 at the center (i.e.,forming a quarter-wave open stub on one side—the left side in theillustration of FIG. 7B—of the graphene sheet 120) may result in theformation of standing waves having a current antinode 840 (and a voltagenode) at or near the graphene sheet 120. The overlap of the currentantinode 840 and the graphene sheet 120 can result in critical couplingof the received electromagnetic radiation to the bolometer for enhanceddetection efficiency. If the graphene sheet is part of a sandwich (e.g.,between layers of boron nitride), contact between the detectortransmission line 815 and the graphene sheet 120 may be made at theedges of the sandwich, as described above with respect to the contacts330 through which the graphene sheet 120 is connected to the Johnsonnoise measuring circuit 130.

In some embodiments a diplexer may be used to separate the receivedmicrowave electromagnetic waves and the Johnson noise signals sent tothe Johnson noise measuring circuit 130. In such an embodiment, thegraphene sheet 120 may be connected to a detector transmission line 815(e.g., in the manner illustrated in FIG. 7B). In such an embodiment, theother two connections to the graphene sheet 120 (shown in FIG. 7B),through the contacts 330, as well as the contacts 330 themselves, may beabsent. The diplexer may have a common port connected to the graphenesheet 120 via the detector transmission line 815, a high frequency (HF)port connected to the input transmission line 805, and a low frequency(LF) port connected to the Johnson noise measuring circuit 130. Thediplexer may separate signals according to frequency, transmittingreceived electromagnetic waves from the input transmission line 805(connected to the high frequency port of the diplexer) to the graphenesheet 120 (through the common port of the diplexer, via the detectortransmission line 815), and transmitting Johnson noise received (at thecommon port of the diplexer, via the detector transmission line 815)from the graphene sheet 210 to the Johnson noise measuring circuit 130(connected to the low frequency port of the diplexer).

Referring to FIG. 8A, in one embodiment a log periodic antenna includestwo conductive patches 910, and may be used to couple microwaveelectromagnetic radiation propagating in free space into a graphenesheet 120, at the center of the log periodic antenna, which may becoupled to a Johnson noise measuring circuit 130. Referring to FIGS. 8Band 8C, the two conductive patches 910 may extend, at the center of thelog periodic antenna, along two sides (e.g., the long sides) of arectangular graphene sheet 120, which may have on its other two (short)sides contacts 330 (not shown in FIG. 8B) through which the graphenesheet 120 is connected to the Johnson noise measuring circuit 130.Referring to FIG. 8C, in another embodiment the two conductive patches910 may extend, at the center of the log periodic antenna, along theshort sides of a rectangular graphene sheet 120, which may have on itsother two (long) sides contacts 330 through which the graphene sheet 120is connected to the Johnson noise measuring circuit 130. As in theembodiments of FIGS. 7A and 7B, the embodiments of FIGS. 8A, 8B, and 8Cmay include additional circuitry (not shown in FIGS. 8A, 8B, and 8C)including chokes to isolate the received electromagnetic waves from theJohnson noise measuring circuit 130. The embodiments of FIGS. 8A, 8B,and 8C may be suitable for detecting electromagnetic radiation in afrequency range extending from about 100 GHz to about 1 THz.

Although limited embodiments of a graphene-based bolometer have beenspecifically described and illustrated herein, many modifications andvariations will be apparent to those skilled in the art. Accordingly, itis to be understood that a graphene-based bolometer employed accordingto principles of this invention may be embodied other than asspecifically described herein. The invention is also defined in thefollowing claims, and equivalents thereof.

What is claimed is:
 1. A bolometer comprising: a graphene sheet having afirst pair of contacts and configured: to be coupled to receivedelectromagnetic waves; to have a temperature, when electromagnetic powerin the received electromagnetic waves is absorbed by the graphene sheet,corresponding to the amount of electromagnetic power absorbed by thegraphene sheet; and to generate thermal noise at the first pair ofcontacts at a level corresponding to the temperature a Fabry-Perotresonator comprising two mirrors, the graphene sheet being between thetwo mirrors; and a circuit connected to the first pair of contacts, thecircuit configured to measure the thermal noise level.
 2. The bolometerof claim 1, further comprising a refrigerator configured to cool thegraphene sheet to a temperature below 4 K.
 3. The bolometer of claim 2,wherein the refrigerator is a pulse tube refrigerator.
 4. The bolometerof claim 2, wherein the refrigerator is a Gifford-McMahon cooler.
 5. Thebolometer of claim 1, wherein the graphene sheet substantially has theshape of a rectangle, the rectangle having a length and a width, thelength being greater than or equal to the width.
 6. The bolometer ofclaim 5, wherein the length of the rectangle is less than 20 microns. 7.The bolometer of claim 5, wherein the product of the length of therectangle and the width of the rectangle is less than 1000 squaremicrons.
 8. The bolometer of claim 5, wherein the graphene sheet has anelectron mobility of more than 100,000 cm²/V/s.
 9. The bolometer ofclaim 1, comprising a first layer of hexagonal boron nitride immediatelyadjacent to a first surface of the graphene sheet, and a second layer ofhexagonal boron nitride immediately adjacent to a second surface of thegraphene sheet.
 10. The bolometer of claim 9, wherein each of the firstlayer of hexagonal boron nitride and the second layer of hexagonal boronnitride has a thickness greater than 4 nm and less than 40 nm.
 11. Thebolometer of claim 1, wherein the circuit comprises an amplifierconnected to the first pair of contacts.
 12. The bolometer of claim 11,further comprising a matching circuit connected between the first pairof contacts and the amplifier.
 13. The bolometer of claim 11, furthercomprising a power detector connected to the amplifier.
 14. Thebolometer of claim 1, wherein the graphene sheet consists of a singleatomic layer of graphene.
 15. The bolometer of claim 1, wherein thegraphene sheet comprises two atomic layers of graphene.
 16. An imagingsystem comprising: an array of bolometers, each of the bolometers of thearray being a bolometer according to claim 1 and having anelectromagnetic wave input, the electromagnetic wave inputs forming anarray of electromagnetic wave inputs; and an imaging system configuredto project an image onto the array of electromagnetic wave inputs.
 17. Abolometer comprising: a graphene sheet having a first pair of contactsand a second pair of contacts and being configured: to be coupled toreceived electromagnetic waves; to have a temperature, whenelectromagnetic power in the received electromagnetic waves is absorbedby the graphene sheet, corresponding to the amount of electromagneticpower absorbed by the graphene sheet; and to generate thermal noise atthe first pair of contacts at a level corresponding to the temperature;and a circuit connected to the first pair of contacts, the circuitconfigured to measure the thermal noise level, wherein a first contactof the second pair of contacts, the graphene sheet, and a second contactof the second pair of contacts together form a part of a microstriptransmission line.
 18. The bolometer of claim 17, wherein: a first endof the microstrip transmission line is coupled to an antenna, and: asecond end of the microstrip transmission line is coupled to ground, ora portion of the microstrip transmission line, including a second end ofthe microstrip transmission line, forms a quarter-wave open stubconnected to the graphene sheet.
 19. A bolometer comprising: a graphenesheet having a first pair of contacts and a second pair of contacts andbeing configured: to be coupled to received electromagnetic waves; tohave a temperature, when electromagnetic power in the receivedelectromagnetic waves is absorbed by the graphene sheet, correspondingto the amount of electromagnetic power absorbed by the graphene sheet;and to generate thermal noise at the first pair of contacts at a levelcorresponding to the temperature; and a circuit connected to the firstpair of contacts, the circuit configured to measure the thermal noiselevel, wherein each contact of the second pair of contacts is connectedto a respective conductor of a pair of conductors of a log periodicantenna.